1. Field of the Invention
This invention relates to wireless communication systems and more particularly to a method for effective wireless communication in the presence of fading and/or other degradations.
2. Description of the Related Art
The capacity and data rate of existing code division multiple access (CDMA) systems can be increased by using multiple antennas at the transmitter. The need to provide increased data rates to a large number of users is an especially urgent problem in both the downlink (base-to-mobile) and the uplink (mobile-to-base) in third generation (3G) wideband CDMA systems. Antenna arrays have been often proposed as a means to improve performance in both links of CDMA systems. In the uplink, an antenna array can be employed at the base station (BS) to provide array gain, interference reduction, and diversity gain. (See “Space-time processing for wireless communications,” by A. Paulraj and C. Papadias, IEEE Signal Processing Magazine, Vol. 14, pp. 49-83, November 1997.) This can, in turn, provide improved range, quality, and capacity in the reverse link of the system.
Since BSs can accommodate the electronics, power consumption, and size of antenna arrays, receive diversity techniques are easily implemented. However, similar improvements on the downlink seem to be harder to obtain. The physical demands of antenna-array processing units make the use of multiple antennas at the mobile handset problematic. The small size of handheld units limits both the spatial resolution of the array (because of the small number of elements), as well as the diversity gain (because the elements are close to one another). It therefore seems more feasible to perform forward-link spatial processing at the BS transmitter.
One possible approach for antenna-array transmit processing is by beamforming which provides array gain at the subscriber unit. In these schemes, the transmitter typically operates in “closed-loop,” i.e., it uses channel information that is fed to it by the receiver through the reverse link in order to shape beams in the forward link (base-to-mobile). The success of transmit beamforming depends on the quality of the channel estimates, the feedback channel, the mapping between the two links, and the dynamics of the signal and interference. Closed-loop techniques typically suffer from reduced reverse link capacity because of the extra channel information that is transmitted.
Another approach employs transmit diversity or orthogonal space-time coding (STC) at the base station with the goal of providing diversity gain to the mobile subscriber. Transmit diversity can be simpler to implement because it can operate in an open-loop, that is, without channel knowledge at the transmitter. This mode of operation is particularly appealing when the mobile speed is high enough to make channel estimation and tracking too difficult. Moreover, open-loop techniques do not penalize the reverse link capacity as closed-loop techniques do. These arguments suggest that multiple-antenna open-loop transmit diversity is a practical way to improve the performance of current systems.
Some open-loop transmit diversity techniques for the CDMA forward link are disclosed in “A comparison of base station transmit diversity methods for third generation cellular standards,” by K. Rohani, M. Harrison and K. Kuchi, in Vehicular Technology Conference, 1999 IEEE 49th, Volume 1, 1999, pages 351-355; “Performance analysis of CDMA transmit diversity methods,” by L. M. A. Jalloul, K. Rohani, K. Kuchi and J. Chen, Vehicular Technology Conference, 1999, VTC 1999, Fall, IEEE VTS 50th, Volume 3, pages 1326-1330; and in “Diversity for the direct-sequence spread spectrum system using multiple transmit antennas,” by V. Weerackody, AT&T Tech. Memo., 1993. In Rohani et al, the spatial diversity inherent in the channel is not fully exploited. This drawback can be compensated when the mobile travels quickly since the receiver gains diversity through temporal fluctuations in the channel (exploited through coding and interleaving). However, when the mobile is stuck in a deep fade on a slow-fading channel, any temporal diversity advantage is lost. Since many 3G wireless data users may be either static or moving at low (pedestrian) speeds, dependence on methods that require temporal diversity, wherever possible, should be reduced. In Weerackody, full spatial diversity is achieved with two transmitter antennas, at the cost, however, of doubling the resources used (either a 100% increase in bandwidth or number of required spreading codes per user). These extra resources limit the overall efficiency of the system.
With closed-loop beamforming, the channel characteristics are measured and the gain and phase of the signals applied to each element in an antenna array are modified to create an antenna pattern that maximizes the power delivered to the mobile station. A disadvantage of this method is the need for constant measurement and feedback of the channel characteristics and the subsequent recalculation of the adaptive array weights used to modify signals for each antenna element. The time needed to measure and compute the weights limits the speed at which the antenna pattern may be modified to compensate for a changing channel. When a mobile station travels at a higher speed, the channel changes at a rate that is higher than the rate of compensation in the adaptive antenna array. Thus, the feedback loop in the adaptive array or beamforming technique cannot keep up with a quickly changing channel between the base station and a high-speed mobile station.
From the above, it becomes apparent that beamforming is desirable when the mobile station is moving into low speeds and orthogonal or space time transmit diversity (STTD) or variations thereof is desirable when the mobile station is moving at high speeds.
Therefore, it is apparent that there is a need for an improved method of transmitting and receiving a traffic channel using technique from both beamforming and orthogonal transmit diversity.